Gastropods are snail-like and slug-like invertebrate (lacking a backbone) animals, and are types of mollusks. Snails have hard mineral shells; slugs lack shells. Because fossils mostly represent the hard parts of organisms, snails are the most common types of gastropod fossils. Slugs are not preserved as fossils. Snails have a single, coiled shell. Modern snails include some of the most common seashells on beaches today, including conch, cowrie, limpet, olive, murex, turban, turret, and whelk shells. Some snails are carnivorous predators, some are herbivores, some are detritus feeders, and others are scavengers. Different species live in fresh, brackish, and marine water. Modern gastropods mostly live on a substrate (sediment or other objects) or burrow in the substrate.
Many gastropod fossils are not fossils of the actual gastropod shell. Most living gastropod shells (and most other mollusks with shells) are composed of the mineral aragonite. Aragonite is a carbonate mineral that is stable at the earth's surface, but dissolves and is replaced by other minerals during burial and fossilization. In some cases, the minerals in buried snail shells are replaced.
In other cases, when gastropod shells dissolve, only casts and molds of the shells remain. Sometimes fossil gastropod casts and molds are of the outside of the original shell, and other times, they are of the inside of the shell.